The present invention is described with particular reference to the treatment of anode slimes produced in the electrolytic refining of copper. Such slimes are a primary source of commercial selenium.
In the electrolytic refining of copper a relatively impure copper is made the anode in an electrolytic cell. As the anode corrodes, copper goes into solution and selenium, tellurium and other elements that do not dissolve in the electrolyte sink to the bottom of the cell and are collected as slimes. The slimes are treated for their valuable components. While copper anode slimes vary widely in composition, they generally contain significant amounts of copper, selenium, tellurium, lead, silver, gold and some platinum group metals. Typical compositions of copper refinery slimes are given on pages 34-35 of SELENIUM edited by Zingaro, R. A. and Cooper, W. C., Van Nostrand Reinhold Company (1974). Approximate ranges of selenium, tellurium, copper, nickel, lead, and precious metals are as follows:
______________________________________ Approx. wt.% Metal Values ______________________________________ 2.8 to 80 copper &lt;1 to 45 nickel 0.6 to 21 selenium 0.1 to 13 tellurium &lt;1 to 45 silver 0.3 to 33 lead up to 3 golo minor amounts platinum group metals ______________________________________
Gangue components such as Al.sub.2 O.sub.3, SiO.sub.2 and CaO are present in the amount of about 2 to 30%.
From the above it can be seen that a considerable amount of copper and tellurium may be present in the copper refinery slimes. The anode slimes may be pre-treated, i.e. before carrying out the steps of the present process for separating out copper and possibly tellurium. Methods for reducing the copper and/or tellurium levels are known. One method consists, for example, of treating the anode slimes with concentrated sulfuric acid at 150.degree.-250.degree. C. to convert copper and tellurium into water soluble compounds. Another consists of selectively extracting copper and tellurium by leaching in a dilute sulfuric acid medium under oxygen pressure. Ferric sulfate is employed in Japan for solubilizing copper.
Whether a pre-treatment step is employed depends on many factors, e.g., the composition of the anode slimes, the purity of the product desired, and the efficiency of a particular plant operation. For example, if the levels of copper and tellurium in the copper refinery slimes are very low and a decopperizing/detellurizing step may not be necessary. If physical separation of precipitates such as copper hydroxide formed during the process is efficient, the need for decopperizing the slimes is reduced. The present process has been carried out effectively for the production of highly pure selenium with as much as, for example, about 11% copper and 2.5% tellurium in the slimes. If a preliminary purification step is performed, the particular sulfation or other decopperizing and/or detellurizing method used is not critical with respect to the process described herein for the recovery of selenium.
In the present process described below, it is assumed that if a decopperizing and/or detellurizing step is carried out, it has already taken place.
A principal objective of the present process is to provide a process by which selenium is extracted and readily separated from anode slimes. A further objective is to recover selenium in elemental form as a commercial grade product. Another objective is to provice a precious metal feed, substantially free of selenium and tellurium for further processing in a Dore furnace prior to recovery of precious metals.
As will appear in more detail in the description below and in the accompanying drawing, the present process involves a caustic oxidative pressure leaching to extract selenium from the slimes in the hexavalent form.
In the description of the process herein, constituents levels are all given in weight percent, and the terminology preponderant or preponderant amount means more than 50%.